when do you replace capacitors?

I have read that capacitors last around 20 years. So, do you just have caps replaced as they go bad, or at what point do you replace all the caps in an amp? Do resistors or other parts (other than tubes) degrade, so replacing caps will just lead to some other failure? Is it like trying to keep an old car running?

Asking because I still enjoy the tube amp (Baron) I've had for 16 years. Not the best, but American made, still supported by Mesa, and I am not convinced that newer is better re: tube amps, or at least, not within semi-rational budgetary constraints. Equivalent performance from new production could cost dearly.